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I have been told there was two Petermann Ranges, the one that produced Uluru and Kata Tjuta has eroded away and no longer can been seen. Is this true? —Kelly

Uluru is easily the most iconic natural landform in Australia, and its formation was equally special. The creation of Uluru and Kata Tjuta — as both were formed at the same time — began over 500 million years ago.

At this time the big crustal blocks that form the Australian continent coming together. A block called the Musgrave Province pushed up from the south creating mountains — the Petermann Ranges — in an event called the Petermann Orogeny (an orogeny is a mountain-building event.)

When they first formed, the Petermann Ranges were a high mountain range more like the Alps or the Himalayas. Today, we can only see the 'nubs' or 'roots' of this once mighty range, says Dr Marita Bradshaw, a geologist with Geosciences Australia.

"The whole landscape was very different that far back, there were no land plants plus we think the climates at that time, after a series of ice ages, may have been desert climate," says Bradshaw.

Without any plant cover the newly formed Petermann Ranges eroded rapidly.

"We had these high mountains with granite outcropping and shedding of conglomerates which ended up being the rock type that we see at Kata Tjuta."

The sediments that make up Kata Tjuta were moved by a river system into an alluvial fan, where they were eventually buried under an inland sea.

However, the feldspar sand that became the arkose sandstone of Uluru was dumped at the bottom of the mountain range.

"What we see in Uluru is almost just shedding of granite," says Bradshaw.

"It's got the big feldspar crystals in it which means it was very close to the source and it hadn't been in a big river system being shifted for a long time, so not much chemical or mechanical erosion happened to it."

The Amadeus Basin

After this period of rapid mountain building and erosion the centre of Australia turned into an inland sea — a tropical seaway went through Central Australia and out through the Kimberley — and a phase of deposition began in what's now known as the Amadeus Basin.

"There was limestone and sand and mud deposited in the Amadeus Basin and that buried the arkose and conglomerate that eventually formed Uluru and Kata Tjuta," says Bradshaw.

At around 400 million years ago the sands and gravels of Uluru and Kata Tjuta were so far down that they were well lithified or knitted together, changing from sediment into rock.

Another mountain-building event — the Alice Springs Orogeny — began around this time, folding the rock and further compressing them.

"What that does is really push down and fold the rocks that become Uluru and Kata Tjuta."

Over millions of years the Alice Springs Orogeny created the great big folds that are visible when you fly over Central Australia today, and in the process further folded and turned the rocks that make up Uluru and Kata Tjuta.

Comments (5)

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Tobias Husband :

20 Nov 2013 10:50:50pm

What a wonderful explanation, it makes my explanation of an "exposed seam of sedimentary rock" seam (pun) pretty superficial. Quite interesting to think of sedimentary rock being "hard" and resistant to weathering, it is truly a special geological icon.

Geoscience Australia :

05 Dec 2013 10:25:37am

The Pine Mountain monolith is granite and not unique. There are other good examples of other granite domes at Bald Mountain in southern Queensland and also Mt Jukes near Mackay, Queensland, which mostly seem to have formed by preferential weathering, i.e, the more resistant units tend to form positive topography - a feature enhanced if the rocks around them are readily prone to weathering (i.e, erosion).

There are probably also local effects, such as finer grain size, lesser jointing, more quartz-rich mineralogy, more homogeneous compositions, that probably also play some role in localising where such granite domes/monoliths form.

Uluru, although being a sediment, is apparently relatively homogeneous, and lacks much jointing or parting, so the overall reasons for both monoliths are probably broadly similar.

Uluru has vertical bedding and variable weathering which gives it its ribbed appearance (10 - 50 cm scale), but on the bigger scale the arkose body is tightly welded together, homogeneous and coherent like a granite.

It makes an impression because it is un-vegetated, other monoliths in Australia may be larger (for example Mt Augustus in Western Australia but do not make the same visual impression.

Some other interesting background information on significant rock features around Australia including Uluru, NT; Mount Augustus WA and Bald Rock on the NSW/QLD border can be found on Geoscience Australia's website (www.ga.gov.au)